In television receivers having both RF and auxiliary (baseband) video inputs it is common practice to clamp the detected and auxiliary signals to similar levels to avoid picture flashing when switching between different video sources. Filliman et al., for example, in U.S. Pat. No. 4,660,084 entitled TELEVISION RECEIVER WITH SELECTABLE VIDEO INPUT SIGNALS describes a system in which both the RF video source and the auxiliary video source are stripped of their original DC values and clamped to new DC reference voltage. This arrangement prevents picture Hashing when changing between sources but employs a pair of feedback controlled keyed clamps which are relatively costly and complex to implement.
FIG. 2 herein shows an arrangement which is exemplary of a prior art arrangement that is relatively simple in that the auxiliary input signal is clamped via a sync tip clamp to approximately the DC level of the detected video input signal. As shown, the system includes an IF input terminal 201 which receives an IF signal from the receiver tuner. Terminal 201 is connected to an IF AGC loop 220 comprising an IF amplifier 202, a video detector 204, an AGC amplifier 206, an AGC smoothing capacitor 208 and an AGC reference voltage source 207. The AGC loop adjusts the gain of the IF amplifier, so that the DC level of the video detector output equals the AGC reference voltage provided by the reference voltage source (illustrated as a battery) 207. The resultant DC stabilized video signal is supplied to one input of a video switch 212 which receives the auxiliary video input signal via capacitor 214 and supplies a selected video output signal. The auxiliary video input signal is clamped by means of a clamp diode 216 and another reference voltage source 218 that is selected to be a level approximating the DC level of the detected video output signal.